Combustion process for coal fired boilers



Aug. 26, 1969 w L Y, JR 3,463,599

COMBUSTION PROCESS FOR COAL FIRED BOILERS Filed March 1, 1967 OXYGEN-FREE FLUE GAS OXYGEN ANALYZER 9 STEAM STEAM i WATER I8 29 17 {I FUEL-I c -:I=::::::

7 COAL- E 9 I 10 E F AIR A. B. WELTY, JR. mvzmun wwy,

PATENT ATYDRNEY United States Patent US. Cl. 431-2 4 Claims ABSTRACT OF THE DISCLOSURE Coal is burned under conditions in which a flue gas is produced containing substantially no oxygen or sulfur trioxide by burning the coal in the presence of excess air and then adding a sufiicient amount of a liquid or gaseous hydrocarbon to the resulting flue gas to burn the excess oxygen.

This invention relates to a process for the combustion of coal. More particularly the invention relates to a process for the combustion of coal under excess air conditions in which the excess oxygen is removed from the flue gas resulting from essentially complete combustion of the coal. Specifically excess oxygen is removed from flue gas by combustion with a fluid hydrocarbon fuel under continuously regulated conditions.

The ideal arrangement for firing fuels is stoichiometric combustion which requires burning the fuel with the precise amount of air theoretically required to burn all of the fuel. If this were possible the flue gas would contain little or no smoke or sulfur trioxide and a bare minimum of nitrogen oxides. Furthermore, boiler tube fouling, heat losses, soot and corrosion would be minimized. However ideal conditions are not attainable with conventional burners and with some fuels such as coal.

The predominant source of fuel for utility boilers and industrial furnaces is coal. Even though the coal is burned in pulverized form in specially designed burners, from 5-30% excess air must be provided to achieve satisfactory combustion and heat release from this fuel. Because of the huge fuel requirements of utility boilers and industrial furnaces the poorer, less expensive grades of coal are used whenever possible and a high ratio of excess air is required.

The object of this invention is to provide a method of mitigating the eflfects of using a high ratio of excess air in coal combustion. Another object of the invention is to prepare flue gas from coal combustion for removal of its S0 content by flue gas desulfurization techniques. In keeping with the latter object another object of the invention is to provide a method of consuming the excess oxygen in the flue gas to place the flue gas slightly on the reducing side.

Broadly summarizing the objects of the invention are achieved by injecting an automatically controlled quantity of a fluid hydrocarbon fuel into the oxygen-containing flue gas resulting from coal combustion and burning the excess oxygen in the flue gas.

The invention will be more fully described below in conjunction with the drawing in which the figure is a schematic sectional view of a boiler disclosing various equipment elements employed in the process of the invention.

In the drawing reference numeral 1 refers generally to an upright boiler of conventional design. The boiler may be of any conventional type designed to produce about 100,000 to 3,000,000 lbs. of steam per hour. Boilers of this capacity are disclosed by Stanair, Plant Engineering Handbook, 2nd Ed., 1959, pp. 12-60 to 12-91. The structural features of the boiler, including the placement ice of the tubes within the various sections, the number and type of burners, etc., do not constitute any part of the invention. In the embodiment shown, the boiler has a main radiant heating section 2 and a convection or superheating section 3. Water is passed by line 4 into boiler tube 5. The water is converted into steam as it passes through the radiant section and the superheat section. Superheated steam is recovered from the boiler by line 6. In practice a multiplicity of boiler tubes is used.

Coal from pipe 7 is pulverized in pulverizer 8 and the pulverized coal is passed by pipe 9 to each of three coal burners 10, 11 and 12. Air is passed by pipe 13 into an air manifold shown generally by reference numeral 14. The coal burners and the air manifold are adapted to provide the proper ratio of coal particles and excess air to the combustion zone which comprises most of the free space in radiant heating section 2. From 5-30 preferably from 10-25% excess air is employed in the combustion of the coal.

The flue gas in the area designated by reference numeral 15 ordinarily contains from 1 to 6 vol. percent based on the flue gas of oxygen. A typical flue gas derived from the combustion of coil contains the above mentioned quantity of oxygen and 1000-4000 ppm. of S0 5-100 p.p.m. of S0 and the balance is N CO H O, NO, fly ash and unburned hydrocarbons.

As mentioned previously in this disclosure it may be desirable to tailor the flue gas so that it is a particular amount on the reducing side. Excess air is consumed according to the following reaction:

Then, the following reactions can be carried out with about two-thirds of the $0 to produce H S 30 3H 9 21-1 0 +H S Finally, the H 8 is reacted with the remaining S0 in the Claus reaction to produce sulfur.

Referring again to the drawing, an auxiliary combustion unit designated generally by reference numeral 16 is located in the area between the radiant zone 2 and the superheat zone 3. In this area combustion of the coal is essentially complete. A fluid fuel is supplied to line 17. The fuel may be a gas, a vapor or a liquid. Suitable fuels include methane, propane, natural gas, refinery gas streams, naphtha, kerosene, and fuel oils. When liquids such as heavy fuel oils are used they are preferably mixed with steam and injected into the combustion area through a steam atomizing nozzle. Water or steam is passed by line 18 into jacket 19 and mixed with the fuel in mixer 20. The mixture of fuel and steam is passed into manifold 21 and injected into the excess air area 15 by nozzles 22 and 23.

An important aspect of the inventio lies in controlling the ratio of auxiliary fuel to excess air so that complete combustion of the excess oxygen is achieved. To this end the boiler stack 24 contains a probe 25 which is connected to an oxygen analyzer 26 by line 27. The flue gas is continuously analyzed for oxygen content and suitable signals are passed by conduit 28 to feed control valve 29. Typically the oxygen content of the flue gas is controlled at about 0.5 mole percent. However, higher or lower levels can be established. If desired, the system can be adjusted to provide a deficiency of oxygen and in this case the flue gas would be analyzed for combustibles such as CO or H EXAMPLE In a typical example the oxygen in the exit flue gas is controlled to 0.1% by volume. The furnace fires tons per hour of pulverized coal using 15% excess air to effect good combustion. The oxygen analyzer controls the oil injection to eliminate all but 0.1% of the Oxygen by volume based on the flue gas. Although the instantaneous amount of oil injected varies with fluctuations in the coal burning, on the average it is found that 65 barrels per hour of fuel oil are required.

By employing the process of the invention it is possible to use less air for a given heat release with consequent savings in compressors and power and also reducing the size of flue gas desulfurization equipment where this is required. The low oxygen content in the flue gas will reduce both high and low temperature corrosion. Fluctuations in the coal burn will be picked up by the oxygen analyzer and corrective action will be taken immediately to assure that all of the excess oxygen is burned by the auxiliary firing system.

What is claimed is:

1. A method for burning coal containing sulfur for eliminating the production of S comprising the steps of first burning the coal at an air fuel ratio greater than that theoretically required for burning under conditions corresponding to stoichiometric conditions, whereby a flue gas containing excess oxygen is obtained, adding to said flue gas an amount of a fluid hydrocarbon suflicient to make the air-fuel ratio substantially equal to the ratio theoretically required for burning at stoichiometric conditions whereby products of combustion corresponding to burning at stoichiometric proportions of air and fuel are obtained which are substantially free of oxygen and S0 2. The process of claim 1 wherein the coal is burnt in the presence of 530 wt. percent excess air.

3. The process of claim 2 wherein the fluid hydrocarbon is natural gas.

4. The process of claim 2 wherein the fluid hydrocarbon is fuel oil mixed with steam.

References Cited UNITED STATES PATENTS 1,665,344 4/1928 Caracristi.

1,971,787 8/1934 Koestner et al --22 3,228,451 1/1966 Fraser et al. 158117.5 3,241,597 3/1966 Juzi 158-1175 JAMES W. WESTHAVER, Primary Examiner 

